Photopolymerizable monomers used in photopolymerizable inkjet inks are, for example, (meth)acrylic acid ester compounds, acryl amide compounds, cation polymerizable oxetane compounds and epoxy compounds. Among them, preferred are (meth)acrylic acid ester compounds and acryl amide compounds since they are available in large amounts, inexpensive, easily procurable, and capable of being used in combination with a radical polymerizable photopolymerization initiator that is inexpensive and commonly used among polymerization initiators.
However, many of (meth)acrylic acid ester compounds and acryl amide compounds have toxicity. Although some of them are relatively mild in oral toxicity and skin irritation, those with low viscosity used in inkjet inks are not safe materials in skin sensitization where they come into contact with the skin to cause allergy.
The present inventor conducted extensive studies and previously found some materials safe in skin sensitization. As a result of further studies, the present inventor previously found that there were some (meth)acrylic acid ester compounds and acryl amide compounds usable as photopolymerizable monomers and having a Stimulation Index (SI value) of less than 3 where the Stimulation Index indicates the extent of sensitization as measured by a skin sensitization test based on the LLNA (Local Lymph Node Assay). However, when those compounds are formulated so as to attain practically usable curing property, the curing property of the resultant inks becomes inferior to those of commonly used photopolymerizable inkjet inks. Thus, a photoradical polymerization initiator has to be incorporated in a larger amount than usual. Such a large amount of the photoradical polymerization initiator incorporated was found to prevent the resultant inks from showing sufficient storage stability in high-temperature environments such as summer.
Also, the present inventor previously found a photopolymerizable inkjet ink which is negative for skin sensitization, improves the coated film in curing property, enables possible curing failures in the coated film to be visually confirmed, and facilitates quality control in the production process. Specifically, the photopolymerizable inkjet ink contains: one or more types of photopolymerizable monomers each having a SI value of less than 3; a self-cleaving photopolymerization initiator; a hydrogen-abstracting polymerization initiator; and an amine compound serving as a polymerization accelerator. However, even this photopolymerizable inkjet ink is difficult to attain sufficient storage stability as described above.
As has widely been known, the storage stability of the photopolymerizable inkjet ink can be improved by the addition of a material called a polymerization inhibitor. Examples of known useful polymerization inhibitors include phenol compounds and quinone compounds such as methoquinone and benzoquinone, and aromatic secondary amine compounds such as diphenyl amine and phenothiazine (see, for example, PTLs 1 and 2).
However, even when such a known polymerization inhibitor as methoquinone is added to the above newly found compound, it is not possible to prevent the resultant inks from thickening or solidification resulting from polymerization. In addition, it was found that aromatic secondary amine compounds such as phenothiazine cannot be used due to their considerable coloring properties, although they have higher polymerization inhibiting effects
Furthermore, many of the photopolymerizable monomers having a SI value of less than 3 have high viscosity as inkjet inks, and the addition of a polymerization initiator increases the inks in viscosity. Thus, it is important to take proper measures to reduce the inks in viscosity. Here, it is easy to reduce their viscosity by incorporating a diluent solvent thereinto. However, use of the diluent solvent is not desirable since it volatizes to be released to the air to give adverse effects to the environment. Therefore, the incorporation of a solvent into inks should be avoided. Alternatively, water may be incorporated into inks containing water-soluble monomers to reduce their viscosity. In this case, when non-permeable base materials such as plastic materials are used, it may not possible to obtain the effect that water penetrates into the non-permeable base materials to lead to drying. Thus, in an attempt to attain high-speed printing process, it is necessary to volatize water instantly for drying. In order to do so, a heat source must be provided in many cases, which is not preferred in terms of energy saving.